1. Field of the Invention
The present invention relates to a method of manufacturing a silicon monocrystal in accordance with a Czochralski (CZ) method in which there are combined a technique of growing a silicon monocrystal without performance of a so-called necking operation and a technique of mechanically holding a part of the crystal during growth thereof.
2. Description of the Related Art
The Czochralski method has been known as a method of manufacturing a semiconductor material such as silicon. As shown in FIG. 3A, according to this method, a seed crystal 52 held by a seed crystal holder 51 is brought into contact with the surface of material melt 54 contained in a crucible 53. The seed crystal 52 is then pulled while being rotated. At this time, the pulling rate and the temperature are controlled such that a neck 55 is formed below the seed crystal 52. This operation is called necking. Subsequently, there is formed a body 56 which is a monocrystal having a large diameter.
The formation of the neck 55 permits elimination of dislocations from the body 56 of the monocrystal formed below the neck 55. In recent years the weight of crystals has increased due to an increase in the diameter of monocrystals and an improvement of manufacturing efficiency, and, pulling of crystals having a weight of even 100 kg or more is now performed. In such a case, the strength of the seed crystal 52 and the neck 55 tends to become insufficient. If the crystal that is being grown falls as a result of fracture of the neck during the course of the pulling of the crystal, a serious accident may occur. In order to prevent such an accident, as shown in FIG. 3B, there has been adopted a method and apparatus which enable mechanical holding of a part of the crystal during growth of the crystal.
In this apparatus, a stepped engagement portion 57 consisting of an increased-diameter portion and a reduced-diameter portion is formed between the neck 55 and the straight body portion 56, and the crystal is pulled while crystal holding jigs 58, 58 hold the stepped engagement portion 57. Examples of such a technique are described in, e.g., Japanese patent Application Laid-open (kokai) Nos. 62-288191, 63-252991, 3-285893, and 3-295893. For instance, in the apparatus disclosed in Japanese patent Application Laid-open No. 3-285893, a stepped engagement portion is formed while a seed crystal is pulled, and when the stepped engagement portion reaches a position corresponding to gripping levers disposed at a predetermined height, the gripping levers grip and pull the stepped engagement portion.
Further, in order to solve the problems, the applicant of the present invention has successfully developed a method of manufacturing a silicon monocrystal, which method can make a growing crystal monocrystalline without performance of necking operation for forming a neck, which would cause a problem in terms of strength, thereby enabling a heavy silicon monocrystal having a large diameter and length to be pulled quite simply while eliminating the necessity of using a complicated apparatus such as a crystal holding mechanism. The applicant of the present invention has also developed a seed crystal used in the method (Japanese patent Application No. 9-17687).
In this method, there is used as a seed crystal a crystal whose tip end portion to be brought into contact with silicon melt has a sharp-pointed shape or a truncation thereof, as shown in FIGS. 2A to 2D. More specifically, the tip end portion of the seed crystal has a tapered shape whose diameter decreases toward the tip end, such as a circular conical shape 7 or a pyramidal shape 8.
A silicon monocrystalline ingot can be grown without formation of a neck, if such a seed crystal is used and pulling operation is performed in such a manner that after the tip end of the seed crystal is gently brought into contact with the silicon melt, the seed crystal is lowered at a slow speed in order to melt the tip end portion of the seed crystal until the thickness of the tip portion increases to a desired value; and subsequently, the seed crystal is pulled slowly in order to grow a silicon monocrystalline ingot having a desired diameter without performance of a necking operation.
Since the above-described method does not include a necking operation, the method can completely solve the problems about fall of the crystal in relation to formation of a neck. Therefore, the above-described method is considerably excellent.
Meanwhile, the above-mentioned method of mechanically holding a part of a crystal being grown has the problem that the part of the crystal cannot be held mechanically until the crystal is grown to a certain degree or length, but in order to prevent fall of the crystal as a result of fracture of the neck, the crystal must be held as early as possible before the crystal is grown to become too heavy.
In order to satisfy the above requirement, the crystal may be held immediately after the engagement stepped portion of the crystal to be held is formed during the growth of the crystal. However, in this case, the crystal must be held right above the material melt, so that a holding device is directly exposed to the high-temperature material melt (the melting temperature of silicon is 1400.degree. C. or higher), thereby resulting in faulty operations or alteration of the quality of the material of the holding device. Further, the material melt may be contaminated by impurities.
If the crystal being grown is subjected to mechanical stress while remaining at elevated temperature, plastic deformation may arise in the crystal, which in turn would cause slip dislocation in the crystal being grown. If such slip dislocation is generated in the crystal being grown, the mechanical strength of the portion of the crystal where the slip dislocation is generated decreases, thereby imposing a risk of fracture for that portion when the crystal has grown to become heavy.
Further, if the crystal is grown to be equal to or greater than a certain weight, the seed crystal or the neck may not bear the weight and hence may fracture. For this reason, it is necessary to grow the crystal to a weight less than the critical weight of the crystal being grown or to mechanically hold the crystal before the weight of the crystal has reached the critical weight.
In growth of a recent large-diameter crystal of, for example, 8 inches or more, the weight of the crystal reaches the critical weight after the crystal is grown to only a slight length. Especially, for the case of crystals having a diameter of 12 to 16 inches which have recently been developed, 1 cm growth of a crystal causes an increase in weight of 1.6 kg to 3 kg. An amount of weight of a crystal ingot becomes 200-300 kg. Therefore, the crystal being grown must be mechanically held as early as possible. In contrast, during the growth of a crystal having a large diameter, a high-temperature region inevitably extends over a wide area of the crystal. Therefore, before the crystal is grown to a certain extent, the temperature of the portion of the crystal to be held does not decrease to a temperature at which plastic deformation does not occur.
Also, in the method in which a part of a crystal is mechanically held, a stepped engagement portion must be formed, resulting in lowered productivity and decreased yield. In addition, forming the stepped engagement portion into a desired shape is difficult, and the formed engagement portion does not necessarily have a shape that can be held.
On the other hand, the CZ method without performance of a necking operation does not cause the above-mentioned problems and therefore can completely solve various problems in relation to formation of the stepped engagement portion.
In recent years, the diameter of monocrystals has been increased due to increased degree of integration of devices, and thus the weight of a crystal to be pulled has increased more than expected. Because of this trend, in addition to a problem in relation to formation of a neck through a necking operation, there have arisen problems of fracture of a seed crystal at a portion where the seed crystal is held by a seed crystal holder, breakage of the seed crystal holder itself, breakage of wire, and the like.